Glamour prism - how to use it

I have the glamour prism and am worried that I will transfer the wrong item to the wrong item. I.e.
When I click the prism it gives me choice to "convert", so does that mean that is the item that I like more than the other one and want to convert that item to my older one?
Or does that mean that I want to convert my old item to a new one?

I am worried to change the wrong item to the wrong thing.

At another spot it says "cast glamour"

So I am thinking maybe I click "convert" on the old item
and click "cast glamour" on the new item
or the other way around?

Click cast glamour in the item you want to be glamoured, then select the item from the list. Make sure you have the right prism for the item. (if the item you wish to take the look of is leather working to repair it will take a leather working prism and so on) Just make sure the item is in your equipment inventory so it will appear.

Whenever you wish to glamour another piece of gear (we'll call it Gear A) to look like another piece of gear (we'll call that Gear B), you select Gear A, and from that submenu click "Cast glamour." A new window will appear listing all the items that you could potentially glamour onto Gear A. You find the item you want Gear A to look like (Gear B), and click on it. Provided you have the right type of glamour prism, and that Gear B can be glamoured onto Gear A (there are some restrictions on glamour), a new window will appear asking if you are sure you want to cast the glamour. You just hit "Cast glamour" again, and viola! Gear A now looks like Gear B!

"Convert" is for converting 100% spiritbonded gear into materia.